One explanation could be the differences in climate (environmental factors). The climate in northern Mexico could support the reproduction of the mosquito species, which is a vector of the dengue virus. Other environmental conditions that influence its endemicity could include living conditions and sanitation. The other possiblity could be that the people in the US have developed immunity for the disease, therefore, are unaffected even when infected by the virus.
Choice C is most likely to be the right answer!
There are some insects that can walk on water, this phenomenon is possible because the water has surface tension. One animal that can walk on water is the lizard.